A general spark plug which is used for igniting an internal combustion engine such as an automotive gasoline engine is comprised of a center electrode, an insulator provided on its outer periphery, a cylindrical metal shell provided on the outer periphery of the insulator, and a ground electrode having a proximal end portion joined to a leading end portion of the metal shell. An externally threaded portion is formed on the outer peripheral surface of the metal shell, and an annular gasket receiving portion projecting in a radially outward direction is formed on a rear end side of the externally threaded portion. Meanwhile, a threaded hole having an internally threaded portion is formed in a cylinder head of the engine. As the externally threaded portion is threadedly secured in that threaded hole, the spark plug is mounted to the engine. Here, the portion on the rear end side of the externally threaded portion of the metal shell adjacent to the gasket receiving portion is a portion called a thread neck, and an annular gasket is provided on that thread neck. As the externally threaded portion is threadedly secured in the threaded hole, i.e., is screwed in, the gasket is compressed between the gasket receiving portion and a peripheral edge portion of the opening of the threaded hole so as to be crushed, thereby sealing the gap between the threaded hole and the gasket receiving portion.
As conventional gaskets, commonplace are those which are obtained by subjecting ring-shaped metallic thin plate members to bending in the radial direction by using a special die unit so as to be formed into predetermined shapes (e.g., so-to-speak hollow shapes having substantially S-shaped cross sections or the like) (for example, refer to JP-A-2001-187966 and the like). After being fitted over the aforementioned thread neck, such a gasket is subjected to predetermined cut bending to thereby form a plurality of (e.g., three) pawl portions in such a manner as to project radially inward. Thus, since the pawl portions are formed after the fitting of the gasket, the gasket is prevented from riding over the thread of the externally threaded portion, thereby preventing the gasket from coming off.
Incidentally, with such as engines of a stratified charge combustion type in recent years, there are cases where if variations occur in an ignition point (i.e., the position within a combustion chamber of a spark discharge gap formed between the center electrode and the ground electrode), an intended form of combustion fails to be obtained. Therefore, in such an engine, it can be said that the angular position (orientation) of the ground electrode is, of course, important, but the position in the vertical direction (axial direction of the plug) of the ignition point in the mounted state of the spark plug is extremely important in ensuring a stable form of combustion.
However, in the case where the above-described gasket having the so-to-speak hollow shape is used, since the amount of crushing deformation at the time of threaded securing is relatively large, the variation of the amount of deformation also becomes large. For this reason, even in cases where the spark plug is mounted to the engine with a predetermined normal torque, the ignition point within the combustion chamber undesirably varies, so that it is apprehended that trouble can possibly occur in achieving stable combustion.
In contrast, it is also conceivable to use a so-to-speak solid gasket with a predetermined thickness and having an annular disk shape (e.g., refer to JP-UM-A-61-57830 and the like). By using such a gasket, the amount of crushing deformation at the time of threaded securing can be made relatively small, and it is possible to suppress the variation of the ignition point. In addition, with such a solid gasket as well, in the same way as described above, it is conceivable to attempt the prevention of coming off by forming a plurality of pawl portions in such a manner as to project radially outward by cut bending after the fitting of the gasket.